2. How Should We View Uncertainty in DD Settings? ∙ Standard approach: all uncertainty in inference enters through sampling error in estimating the means of each group/time period combination. Long history in analysis of variance. ∙ Recently, different approaches have been suggest that focus on different kinds of uncertainty – perhaps in addition to sampling error in estimating means. Bertrand, Duflo, and Mullainathan (2004), Donald and Lang (2007), Hansen (2007a,b), and Abadie, Diamond, and Hainmueller (2007) argue for additional sources of uncertainty. ∙ In fact, for the most part, the additional uncertainty is assumed to swamp the sampling error in estimating group/time period means. (See DL